Selective adherence of stent-graft coverings, mandrel and method of making stent-graft device

ABSTRACT

A method for selectively bonding layers of polymeric material, especially expanded polytetrafluoroethylene (ePTFE), to create endoluminal vascular devices. In a preferred method the selective bonding is achieved by applying pressure to selected areas using a textured mandrel. This permits a stent device to be encapsulated between two layers of ePTFE with unbonded slip pockets to accommodate movement of the structural members of the stent. This allows stent compression with minimal force and promotes a low profile of the compressed device. Unbonded regions of ePTFE allow enhanced cellular penetration for rapid healing and can also contain bioactive substance that will diffuse through the ePTFE to treat the vessel wall.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to patent application Ser. No.08/508,033, filed Jul. 25, 1995, U.S. Pat. No. 5,749,880, which is acontinuation in part of co-pending application Ser. No. 08/401,871,filed Mar. 10, 1995 and to co-pending patent application Ser. No.08/794,871, filed Feb. 5, 1997 and Provisional Application No.60/102,518, filed Sep. 30, 1998, each of which is the subject of commonassignment from the inventors and is hereby incorporated by reference asteaching a method of encapsulating an endoluminal stent device betweenluminal and abluminal polytetrafluoroethylene coverings.

BACKGROUND OF THE INVENTION

[0002] The present invention relates generally to endoluminalstent-graft devices suitable for percutaneous delivery into a bodythrough anatomical passageways to treat injured or diseased areas of thebody. More particularly, the present invention relates to a method ofbonding microporous polytetrafluoroethylene (“PTFE”) coverings over astent scaffold in a manner which maintains unbonded regions to act asslip planes or pockets to accommodate planar movement of stent elements.In one embodiment of the present invention bonded and unbonded regionsare formed by means of a mandrel which has a pattern of either raisedprojections or recesses in its surface which are either synchronous orasynchronous, respectively, with stent elements.

[0003] The use of implantable vascular grafts comprised of PTFE is wellknown in the art. These grafts are typically used to replace or repairdamaged or occluded blood vessels within the body. However, if suchgrafts are radially expanded within a blood vessel, they will exhibitsome subsequent retraction. Further, such grafts usually requireadditional means for anchoring the graft within the blood vessel, suchas sutures, clamps, or similarly functioning elements. To minimize theretraction and eliminate the requirement for additional attachmentmeans, those skilled in the art have used stents, such as thosepresented by Palmaz in U.S. Pat. No. 4,733,665 and Gianturco in U.S.Pat. No. 4,580,568 which patents are herein incorporated by reference,either alone or in combination with PTFE grafts.

[0004] For example, the stent described by Palmaz in U.S. Pat. No.4,733,665 can be used to repair an occluded blood vessel. The stent isintroduced into the blood vessel via a balloon catheter, which is thenpositioned at the occluded site of the blood vessel. The balloon is thenexpanded thereby expanding the overlying stent to a diameter comparableto the diameter of an unoccluded blood vessel. The balloon catheter isthen deflated and removed with the stent remaining seated within theblood vessel because the stent shows little or no radial retraction. Useof radially expandable stents in combination with a PTFE graft isdisclosed in U.S. Pat. No. 5,078,726 to Kreamer. This reference teachesplacing a pair of expandable stents within the interior ends of aprosthetic graft having a length that is sufficient to span the damagedsection of a blood vessel. The stents are then expanded to secure thegraft to the blood vessel wall via a friction fit.

[0005] Although stents and stent/graft combinations have been used toprovide endovascular prostheses that are capable of maintaining theirfit against blood vessel walls, other desirable features are lacking.For instance, features such as increased strength and durability of theprosthesis, as well as an inert, smooth, biocompatible blood flowsurface on the luminal surface of the prosthesis and an inert, smoothbiocompatible surface on the abluminal surface of the prosthesis, areadvantageous characteristics of an implantable vascular graft. Some ofthose skilled in the art have recently addressed these desirablecharacteristics by producing strengthened and reinforced prosthesescomposed entirely of biocompatible grafts and graft layers.

[0006] For example, U.S. Pat. No. 5,048,065, issued to Weldon, et al.discloses a reinforced graft assembly comprising a biologic orbiosynthetic graft component having a porous surface and a biologic orbiosynthetic reinforcing sleeve which is concentrically fitted over thegraft component. The reinforcing sleeve includes an internal layer, anintermediate layer, and an external layer, all of which comprisebiocompatible fibers. The sleeve component functions to providecompliant reinforcement to the graft component. Further, U.S. Pat. No.5,163,951, issued to Pinchuk, et al. describes a composite vasculargraft having an inner component, an intermediate component, and an outercomponent. The inner and outer components are preferably formed ofexpanded PTFE while the intermediate component is formed of strands ofbiocompatible synthetic material having a melting point lower than thematerial which comprises the inner and outer components.

[0007] Another reinforced vascular prosthesis having enhancedcompatibility and compliance is disclosed in U.S. Pat. No. 5,354,329,issued to Whalen. This patent discloses a non-pyrogenic vascularprosthesis comprising a multilamellar tubular member having an interiorstratum, a unitary medial stratum, and an exterior stratum. The medialstratum forms an exclusionary boundary between the interior and exteriorstrata One embodiment of this prosthesis is formed entirely of siliconerubber that comprises different characteristics for the different stratacontained within the graft.

[0008] The prior art also includes grafts having increased strength anddurability, which have been reinforced with stent-like members. Forexample, U.S. Pat. No. 4,731,073, issued to Robinson discloses anarterial graft prosthesis comprising a multi-layer graft having ahelical reinforcement embedded within the wall of the graft. U.S. Pat.No. 4,969,896, issued to Shors describes an inner elastomericbiocompatible tube having a plurality of rib members spaced about theexterior surface of the inner tube, and a perforate flexiblebiocompatible wrap circumferentially disposed about, and attached to,the rib members.

[0009] Another example of a graft having reinforcing stent-like membersis disclosed in U.S. Pat. No. 5,123,917, issued to Lee which describesan expandable intraluminal vascular graft having an inner flexiblecylindrical tube, an outer flexible cylindrical tube concentricallyenclosing the inner tube, and a plurality of separate scaffold memberspositioned between the inner and outer tubes. Further, U.S. Pat. No.5,282,860, issued to Matsuno et al. discloses a multi-layer stentcomprising an outer resin tube having at least one flap to provide ananchoring means, an inner fluorine-based resin tube and a mechanicalreinforcing layer positioned between the inner and outer tubes.

[0010] Still another stent containing graft is described in U.S. Pat.No. 5,389,106 issued to Tower which discloses an impermeable expandableintravascular stent including a dispensable frame and an impermeabledeformable membrane interconnecting portions of the frame to form animpermeable exterior wall. The membrane comprises a synthetic non-latex,non-vinyl polymer while the frame is comprised of a fine platinum wire.The membrane is attached to the frame by placing the frame on a mandrel,dipping the frame and the mandrel into a polymer and organic solventsolution, withdrawing the frame and mandrel from the solution, dryingthe frame and mandrel, and removing the mandrel from the polymer-coatedframe.

[0011] Microporous expanded polytetrafluoroethylene (“ePTFE”) tubes maymade by any of a number of well-known methods. Expanded PTFE isfrequently produced by admixing particulate dry polytetrafluoroethyleneresin with a liquid lubricant to form a viscous slurry. The mixture ispoured into a mold, typically a cylindrical mold, and compressed to forma cylindrical billet. The billet is then ram extruded through anextrusion die into either tubular or sheet structures, termed extrudatesin the art. The extrudates consist of extruded PTFE-lubricant mixturecalled “wet PTFE.” Wet PTFE has a microstructure of coalesced, coherentPTFE resin particles in a highly crystalline state. Following extrusion,the wet PTFE is heated to a temperature below the flash point of thelubricant to volatilize a major fraction of the lubricant from the PTFEextrudate. The resulting PTFE extrudate without a major fraction oflubricant is known in the art as dried PTFE. The dried PTFE is theneither uniaxially, biaxially or radially expanded using appropriatemechanical apparatus known in the art. Expansion is typically carriedout at an elevated temperature, eg., above room temperature but below327° C., the crystalline melt point of PTFE. Uniaxial, biaxial or radialexpansion of the dried PTFE causes the coalesced, coherent PTFE resin toform fibrils emanating from nodes (regions of coalesced PTFE), with thefibrils oriented parallel to the axis of expansion. Once expanded, thedried PTFE is referred to as expanded PTFE (“ePTFE”) or microporousPTFE. The ePTFE is then transferred to an oven where it is sintered bybeing heated to a temperature above 327° C., the crystalline melt pointof PTFE. During the sintering process the ePTFE is restrained againstuniaxial, biaxial or radial contraction. Sintering causes at least aportion of the crystalline PTFE to change from a crystalline state to anamorphous state. The conversion from a highly crystalline structure toone having an increased amorphous content locks the node and fibrilmicrostructure, as well as its orientation relative to the axis ofexpansion, and provides a dimensionally stable tubular or sheet materialupon cooling. Prior to the sintering step, the lubricant must be removedbecause the sintering temperature of PTFE is greater than the flashpoint of commercially available lubricants.

[0012] Sintered ePTFE articles exhibit significant resistance to furtheruniaxial, or radial expansion. This property has lead many in the art todevise techniques which entail endoluminal delivery and placement of anePTFE graft having a desired fixed diameter, followed by endoluminaldelivery and placement of an endoluminal prosthesis, such as a stent orother fixation device, to frictionally engage the endoluminal prosthesiswithin the lumen of the anatomical passageway. The Kreamer patent, U.S.Pat. No. 5,078,726, discussed above, exemplifies such use of an ePTFEprosthetic graft. Similarly, published International Applications No.WO95/05132 and WO95/05555, filed by W.L. Gore Associates, Inc., discloseballoon expandable prosthetic stents which have been covered on innerand outer surfaces by wrapping ePTFE sheet material about the balloonexpandable prosthetic stent in its enlarged diameter, sintering thewrapped ePTFE sheet material to secure it about the stent, and crimpingthe assembly to a reduced diameter for endoluminal delivery. Oncepositioned endoluminally, the stent-graft combination is dilated tore-expand the stent to its enlarged diameter returning the ePTFEwrapping to its original diameter.

[0013] Thus, it is well known in the prior art to provide an ePTFEcovering which is fabricated at the final desired endovascular diameterand is endoluminally delivered in a folded or crimped condition toreduce its delivery profile, then unfolded in vivo using either thespring tension of a self-expanding, thermally induced expandingstructural support member or a balloon catheter. However, the knownePTFE covered endoluminal stents are often covered on only one surfaceof the stent, i.e., either the lumenal or abluminal wall surface of thestent. Where the stent is fully covered on both the luminal andabluminal wall surfaces of the stent, the covering completely surroundsthe stent elements and fills the stent interstices. When theencapsulated stent is comprised of shape memory alloy, characteristicsof the stent make it necessary to encapsulate in the “large” state andthen compress the encapsulated stent for delivery. In this caseencapsulation either increases the device's resistance to compression,or increases the delivery profile of the device as compression causesthe polymeric material to fold or buckle around the stent. Perhaps themost serious problem is that the folding during compression actuallyencompasses folding of the stent itself, which unduly stresses the stentmaterial and may result in structural failure.

[0014] In contrast to the prior art, the present invention provides amethod to encapsulate a stent in ePTFE whereby the structure containspockets or regions where the ePTFE layers are not adhered to one anotherallowing the stent to contract or expand without being encumbered byePTFE and without folding or stressing the stent itself.

[0015] As use herein, the following terms have the following meanings:

[0016] “Fibril” refers to a strand of PTFE material that originates fromone or more nodes and terminates at one or more nodes.

[0017] “Node” refers to the solid region within an ePTFE material atwhich fibrils originate and converge.

[0018] “Internodal Distance” or “IND” refers to a distance between twoadjacent nodes measured along the longitudinal axis of fibrils betweenthe facing surfaces of the adjacent nodes. IND is usually expressed inmicrometers (μm).

[0019] “Node Length” as used herein refers to a distance measured alonga straight line between the furthermost end points of a single nodewhich line is perpendicular to the fibrils emanating from the node.

[0020] “Nodal Elongation” as used herein refers to expansion of PTFEnodes in the ePTFE microstructure along the Node Length.

[0021] “Longitudinal Surface” of a node as used herein refers to a nodalsurface from which fibrils emanate.

[0022] “Node Width” as used herein refers to a distance measured along astraight line, drawn parallel to the fibrils, between opposinglongitudinal surfaces of a node.

[0023] “Plastic Deformation” as used herein refers to the deformation ofthe ePTFE microstructure under the influence of a expansive force whichdeforms and increases the Node Length and results in elastic recoil ofthe ePTFE material less than about 25%.

[0024] “Radially Expandable” as used herein to describe the presentinvention refers to a property of the ePTFE tubular member to undergoradially oriented Plastic Deformation mediated by Nodal Elongation:

[0025] “Structural Integrity” as used herein to describe the presentinvention in terms of the ePTFE refers to a condition of the ePTFEmicrostructure both pre- and post-radial deformation in which thefibrils are substantially free of fractures or breaks and the ePTFEmaterial is free of gross failures; when used to describe the entiredevice “Structural Integrity” may also include delamination of the ePTFElayers.

[0026] Endoluminal stent devices are typically categorized into twoprimary types: balloon expandable and self-expanding. Of theself-expanding types of endoluminal stent devices, there are twoprinciple sub-categories: elastically self-expanding and thermallyself-expanding. The balloon expandable stents are typically made of aductile material, such as stainless steel tube, which has been machinedto form a pattern of openings separated by stent elements. Radialexpansion is achieved by applying a radially outwardly directed force tothe lumen of a balloon expandable stent and deforming the stent beyondits elastic limit from a smaller initial diameter to an enlarged finaldiameter. In this process the slots deform into “diamond shapes.”Balloon expandable stents are typically radially and longitudinallyrigid and have limited recoil after expansion. These stents havesuperior hoop strength against compressive forces but should thisstrength be overcome, the devices will deform and not recover.

[0027] Self-expanding stents, on the other hand, are fabricated fromeither spring metal or shape memory alloy wire which has been woven,wound or formed into a stent having interstices separated with wirestent elements. When compared to balloon-expandable stents, thesedevices have less hoop strength but their inherent resiliency allowsthem to recover once a compressive force that results in deformation isremoved.

[0028] Covered endoluminal stents are known in the art. Heretofore,however, the stent covering has been made of a polymeric material whichhas completely subtended the stent interstices, that is, the stent wascompletely embedded in the polymeric material. This has posed difficultyparticularly with the self-expanding stents. To preserve theirself-expanding property, all covered self-expanding stents have beencovered with a polymeric covering while the stent is in its unstraineddimensional condition, i.e.; its native enlarged diameter. Yet todelivery a covered stent it must be constricted to a smaller deliverydiameter. Radial compression of a stent necessarily causes theindividual stent elements to traverse the stent interstices and passinto proximity to a laterally adjacent individual stent element, therebyoccupying the previously open interstitial space. Any polymeric materialwhich subtends or resides within the previously open interstitial spacewill necessarily be displaced, either through shearing, fracturing orotherwise responding to the narrowing of the interstitial space as thestent is compressed from its enlarged unstrained diameter to itsstrained reduced diameter. Because the struts of the stent arecompletely encapsulated, resistance of the polymer may cause folding orstressing of the struts during compression.

[0029] It was recognized, therefore, that a need has developed toprovide an encapsulating covering for a stent which is permanentlyretained on the stent, substantially isolates the stent material fromthe body tissue forming the anatomical passageway or from matter withinthe anatomical passageway, and which permits the stent to deform withoutsubstantial interference from the covering material.

[0030] It is, therefore, a primary objective of the present invention toprovide a method for encapsulating an endoluminal stent such that theencapsulating covering forms non-adhered regions which act as slipplanes or pockets to permit the individual stent elements to traverse asubstantial surface area of interstitial space between adjacent stentelements without resistance or interference from the encapsulatingcovering, thereby avoiding damage or stress to the stent elements.

[0031] It is a further object of the present invention to use thepockets between the bonded regions to contain and deliver therapeuticsubstances.

[0032] It is another objective of the present invention to provide anapparatus for applying to and selectively adhering sections of theencapsulating covering about the stent, and to provide a selectivelyadhered encapsulated covered stent-graft device.

SUMMARY OF THE INVENTION

[0033] These and other objectives of the present invention are achievedby providing an encapsulated stent-graft device in which an endoluminalstent having a plurality of individual stent elements separated byinterstitial spaces is circumferentially covered along at least aportion of its longitudinal axis by at least one luminal and at leastone abluminal covering of a polymeric material, the luminal andabluminal coverings being selectively adhered to one another at discreteportions thereof in a manner which forms a plurality of open pocketssurrounding a plurality of stent elements. A radially expandablereinforced vascular graft that includes a first layer of biocompatibleflexible material, a second layer of biocompatible flexible material,and a support layer sandwiched between the first and second layers ofbiocompatible flexible material. In addition, the selective bondingsystem disclosed herein can be advantageously used to produce inflatablepockets by bonding the first layer to the second layer in definedpatterns. The resulting structure can then be inflated and stiffened byinjection of a fluid resulting in a supporting structure withoutinclusion of a stent. A crude analogy might be the construction of anair mattress that is composed of flexible polymeric layers bonded toeach other in a predetermined pattern.

[0034] The at least one luminal and at least one abluminal covering of apolymeric material are preferably comprised of expanded PTFE, unexpandedporous PTFE, woven polyester or expanded PTFE yarns, polyimides,silicones, polyurethane, fluoroethylpolypropylene (FEP),polypropylfluorinated amines (PFA), or other related fluorinatedpolymers.

[0035] The stent preferably comprises a stent and may be made of anystrong material which can undergo radial expansion but which is alsoresistant to non-elastic collapse such as silver, titanium,nickel-titanium alloys, stainless steel, gold, or any suitable plasticmaterial capable of maintaining its shape and material properties atsintering temperatures and having the necessary strength and elasticityto enable radial expansion without collapse due to the presence of thepolymer coverings.

[0036] A preferred embodiment of the radially expandable reinforcedvascular device comprises a tubular stent, composed of a plurality ofstent elements and stent interstices, the tubular stent isconcentrically covered along at least a portion of its longitudinallength by a luminal polymeric covering and an abluminal polymericcovering. The luminal and abluminal polymeric coverings arediscontinuously joined to one another through some of the stentinterstices. The luminal and abluminal polymeric coverings may beshorter in length than the stent member to permit opposing stent ends toflare outwardly upon radial expansion of the stent member.Alternatively, the ends of the stent member may be completely encased bythe luminal and abluminal polymeric coverings.

[0037] The stent member is preferably a self-expanding stent, which maybe either an elastic spring material stent, such as a stainless steelstent as disclosed in Wall, U.S. Pat. No. 5,266,073 or a non-wovenstainless steel self-expanding stent as disclosed in Gianturco, U.S.Pat. No. 5,282,824, or a thermoelastic stent made of a shape memoryalloy, eg., a nickel-titanium alloy commonly known as NITINOL, such asthat disclosed in U.S. Pat. No. 5,147,370. Tubular shaped support memberpreferably comprises a stent made of silver, titanium, stainless steel,gold, or any suitable plastic material capable of maintaining its shapeand material properties at sintering temperatures and having thestrength and elasticity to permit radial expansion and resist radialcollapse.

[0038] In accordance with the present invention, selective bonding ofexpanded PTFE luminal and abluminal layers encapsulates the endoluminalstent and isolates the stent from both the tissue forming the anatomicalpassageway as well as any fluid, such as blood, bile, urine, etc. whichmay pass through the anatomical passageway. The presence of slip planesor pockets formed by the selectively adhered regions of ePTFE i) permitsfreedom of movement of stent elements within the encapsulating coveringduring both during expansion and contraction of the stent along eitherits radial or longitudinal axes; ii) permits uniform folding of theePTFE stent covering material which is complementary to the structure ofthe stent element lattice; iii) permits movement of the stent relativeto the ePTFE encapsulating layers; iv) reduces forces required tocompress or dilate the stent in the case of elastically or thermallyself-expanding stents; v) reduces radial expansion pressures required toballoon expand an ePTFE encapsulated stent; and vi) provides voidregions which may be used in conjunction with the microporousmicrostructure of the ePTFE covering material to retain and releasebioactive substances, such as anticoagulant drugs, anti-inflammatorydrugs, or the like.

[0039] Alternative arrangements of the stent member or other suitablestructural support sufficient to maintain the lumenal patency of thelumenal and abluminal polymer coverings may be employed. For example, aradially expandable, articulated reinforced vascular graft may be formedby concentrically interdisposing a structural support assemblycomprising multiple stent members spaced apart from one another betweentwo tubular polymer covering members, then partially joining the twotubular polymer covering members by circumferentially compressingselected regions of the two tubular polymer covering members andthermally bonding the selectively compressed regions to one another.

[0040] The present invention also encompasses selective bonding ofmultiple polymeric layers to create an inflatable structure. Such astructure can be inflated by fluids delivered through lumens within thedelivery catheter. The selective bonding method allows creation ofdevices with multiple adjacent channels or pockets. Some of thesepockets can be prefilled with a therapeutic drug to prevent restenosisor local thrombosis. Alternate pockets can be arranged for fluidinflation after the device is inserted.

[0041] One method of making the foregoing encapsulated stent-graft is tojoin concentrically a luminal polymeric tube, an endoluminal stent, andan abluminal polymeric tube and to place the assembly onto a mandrelhaving a plurality of raised projections separated by land areas, or bya plurality of land areas separated by a plurality of recesses. Eitherthe raised projections or the land areas are patterned to match apattern of either the stent elements of the stent interstices, both thestent elements and stent interstices or portions of each. In this waythe projections or the landed areas exert pressure, respectively onselect regions of the PTFE resulting in limited regions of adherence orfusion when the device is heated to sintering temperatures. With amandrel luminal pressure is selectively applied to produce selectivelyplaced bonds. As will become clear, bonding pressure can be applied fromthe luminal or the abluminal or both surfaces of the device.

[0042] The present invention is also directed to a process for making aradially expandable reinforced stent-graft device by the steps of:

[0043] a) positioning a radially expandable stent member composed of aplurality of interconnected stent elements and a plurality ofinterstices between adjacent interconnected stent elements,concentrically over a first polymeric cover member,

[0044] b) positioning a second polymer cover member concentrically overthe radially expandable stent member and the first polymeric covermember;

[0045] c) selectively joining portions of the first polymeric covermember and the second polymeric cove member through a plurality of theinterstices of the stent member, while leaving portions of the first andsecond polymeric cover members unjoined and forming slip planes orpockets to accommodate movement of at least a portion of theinterconnected stent elements therethrough;

[0046] d) fully joining opposing end regions of the first and secondpolymer cover members through the interstices of the stent memberproximate to opposing ends of the stent member

[0047] The step of fixing the support layer to the biocompatible graftlayers comprises selectively applying pressure to the portions of theluminal and abluminal polymer covers after they are loaded onto amandrel and then heating the resulting assembly at sinteringtemperatures to form a mechanical bond at the selected areas of appliedpressure. Alternatively, a pattern of at least one of an adhesive, anaqueous dispersion of polytetrafluoroethylene, a polytetrafluoroethylenetape, fluoroethylpolypropylene (FEP), or tetrafluoroethylene(collectively the “adhesive”) may be introduced between the luminal andabluminal polymer covers at selected positions, followed by heating theassembly to the melt temperature of the adhesive to bond the luminal andabluminal polymer covers while leaving unbonded slip plane regions toaccommodate movement of the stent elements. If ultraviolet curableadhesives are used, a UV laser or a photolithography system can be usedto create the bond pattern. Many thermoplastic polymers such aspolyethylene, polypropylene, polyurethane and polyethylene terephthalatecan also be used. If pieces of one of these or similar polymers areplaced or attached to one of the polymer covers in the region to bebonded, heat and pressure will melt the thermoplastic causing it to flowinto the pores of the ePTFE, thereby bonding the ePTFE layers together.

[0048] These and other objects, features and advantages of the presentinvention will become more apparent to those skilled in the art whentaken with reference to the following more detailed description of thepreferred embodiments of the invention in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0049]FIG. 1 is a process flow diagram illustrating a preferred methodof making the inventive stent-graft device in accordance with thepresent invention.

[0050]FIG. 2 is a perspective view of a mandrel having longitudinalridges or splines.

[0051]FIG. 3 is a cross-section view of the mandrel shown in FIG. 2.

[0052]FIG. 4 is a perspective view of a stent-graft device illustratingselected regions of bonding between the luminal and abluminal stentcovers and a plurality of slip plane pockets intermediate the lumenaland abluminal stent covers.

[0053]FIG. 5 is a cross-sectional view taken along line 5-5 of FIG. 4.

[0054]FIG. 6 is a scanning electron micrograph illustrating aselectively bonded region and a slip plane pocket with a stent elementresiding therein, of the inventive stent-graft device.

[0055]FIG. 7 is a perspective view of a mandrel having circumferentialridges (as opposed to longitudinal splines).

[0056]FIG. 8 is a flow diagram showing a method of using adhesives tocreate selective adherence.

[0057]FIG. 9 is a flow diagram of an alternative method of usingadhesives to create selective bonds.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0058] The selective adherence encapsulation of the present invention isan improvement of the total adherence method taught in U.S. Pat. No.5,749,880 that is incorporated herein by reference. That patentdiscloses a method for encapsulating a support stent by placing thestent over a first tubular member of unsintered ePTFE and then placing asecond tubular member of unsintered ePTFE coaxially over the stent sothat the stent is sandwiched between two layers of ePTFE. Radial forceis applied either internally or externally to force the first tubularmember into contact with the stent and into contact with the secondtubular members through openings in the stent or, respectively, to forcethe second tubular into contact with the stent and into contact with thefirst tubular member through openings in the stent. Finally, thecompound structure is exposed to an elevated temperature to bond thefirst tubular member to the second tubular member wherever they arepressed into contact. In one embodiment an adhesive spread between thetubular members achieves the bonding. In a preferred embodiment theelevated temperature is a sintering temperature (above the crystallinemelting point of PTFE) and direct PTFE to PTFE bonds form.

[0059] As mentioned above, a potential drawback of this approach is thatwhen the radial dimensions of the stent change, movement of componentsof the stent (necessary for radial dimensional changes) may be impededby surrounding ePTFE. If the stent is encapsulated in an expanded formand then reduced in diameter prior to insertion into a patient, theencapsulating ePTFE may significantly increase the force needed tocompress the stent and may fold in a manner so as to increase theprofile of the collapsed device. If the bonding of the first member tothe second member is selective, i.e., does not occur through allavailable openings in the stent, slip planes or pockets will be left inthe structure so that stent components can reorient within these pocketswithout encountering resistance from the ePTFE. Without the slip planesformed by the selective bonds of the present invention crimping a shapememory stent may cause the stent members to fold or otherwise becomestressed. This can result in permanent damage to the stent.

[0060] There is a considerable possible range of extent for theselective adherence of the instant invention. At one extreme is a fullyencapsulated stent as provided by the '880 patent in which there isfully bonding between all areas of the two tubular members in which thestent struts do not block contact. At the other extreme would be a “spotwelded” device where only tiny areas, probably in the middle of the openareas of the stent structure, are bonded. At that extreme there might bea tendency for the PTFE members to separate from the stent should thespot weld bond strength be exceeded; however, the spot weld structurewould provide virtually no impedance to radial deformation of the stent.

[0061] The optimum extent of selective adherence as well as thegeometric position of the bonds in relation to the stent depends on thestructure of the stent as well as the desired properties of thecompleted device. Complete control of the bond positions can be achievedby a numerically controlled (NC) machine in which the two-ePTFE memberswith the interposed stent are mounted on a mandrel that is attached tothe spindle drive of a modified NC lathe. In this device a heated toolwhose tip is equal to the desired spot weld area is automaticallypressed onto the mandrel-mounted ePTFE-stent sandwich in properregistration to create a bond in an open region between components orstruts of the stent. The tool moves away slightly as the mandrel turnsto expose another open region and the tool then moves in to create asecond bond and so on. Depending on the distance that the mandrel turns,the spot welds may be in adjacent open spaces or may skip one or moreopen spaces. As the mandrel is turned, the tool advances along thelongitudinal axis of the mandrel so that virtually any patterns of spotwelds can be created on the ePTFE-stent device. The precise pattern isunder computer control and an entire stent can be treated quite quickly.If the design calls for spot welds of different surface areas, the stentcan be treated with different tools (e.g., different areas) in severalpasses. An ultrasonic welding tip can readily be substituted for theheated tool. It is also possible to use radiant energy, as with a laser,to effect similar results. However, the inventors presently believe thatpressure as well as heat are needed for the best bonds. Currently,laser-induced bonds do not appear to be as strong as bonds that are madewith heat and pressure unless a curable adhesive system (as with a UVlaser) is employed.

[0062] Splined or textured mandrels can also be used to apply selectiveheat and pressure to create selective adherence between the ePTFEmembers. By “spline or splined” is meant an cylindrical structure withlongitudinally oriented ridges equally spaced about the structure'scircumference. Wherever the first and second ePTFE tubular members comeinto contact a bond can be formed if heat and pressure are applied. Ifthe ePTFE tubular members and support stent are placed over a mandrelwhose surface is patterned with elevated and depressed regions, (hillsand valleys) the elevated regions or ridges will apply pressure to theoverlying stent-ePTFE regions allowing selective bonding of thoseregions. Regions of ePTFE overlying valleys will not be pressed togetherand no bond will form there. That is, the pattern of the mandrel will betranslated into an identical pattern of bonded regions in thestent-graft device. To make this translation the process diagram of FIG.1 is followed.

[0063] In a first step 32, a first ePTFE tubular member is placed on amandrel. Preferably the first tubular member is composed of unsinteredePTFE. In a second step 34, a stent device is placed over the firsttubular member. In a third step 36, a second ePTFE tubular member isslid coaxially over the stent. The second tubular member may beunsintered or partially sintered. Use of a partially sintered secondtubular member reduces the chance of tearing the member while pulling itover the stent. It will be apparent to one of skill in the art thatthere is an advantage to using a second tubular member with a slightlylarger diameter than the first tubular member. However, if the secondtubular member is too large, folds or creases may develop during thebonding process.

[0064] This entire process may use one of the textured mandrels thatwill be described below. However, it is also possible to assemble one orboth tubular members and the stent on a smooth mandrel and then slip theassembly off the smooth mandrel and onto the textured mandrel. If thefit is fairly tight, it may be easier to place the stent over the firsttubular member when that member is supported by a smooth mandrel. Also,there may be a limited number of textured mandrels available forproduction so that making a number of ePTFE-stent assemblies on lessexpensive smooth mandrels may result in a significant savings of time.If a smooth mandrel is used, the stent assembly is transferred to atextured mandrel before the next step (wrapping) occurs.

[0065] In a fourth step 38, the ePTFE-stent assembly is helicallywrapped with PTFE “tape.” This tape is actually a long, thin strip ofPTFE of the type generally known as “plumber's tape.” The tape is evenlywound over the stent device so that the device is covered from end toend. The tape is wound so that the long axis of the tape isapproximately normal (offset by 10-15°) to the long axis of the stentdevice. Ideally, there should be some overlap of the tape covering thedevice so that coverage is even and complete. In fact an overlap ratiowherein five revolutions is needed to progress one tape width has proveneffective. The tape should be applied with a controlled and even tensionso that it is sufficiently tight to apply pressure at right angles tothe surface of the stent device. One way of achieving this is to use aforce clutch on the tape spool to ensure a reproducible tension in thetape as it is wound over the stent device. While this process can beperformed by hand, it is fairly easy to automate the winding process byhaving the mandrel mounted in a modified lathe. As the lathe spindleturns, the spool of tape automatically advances along the turningmandrel ensuring an even and reproducible wrapping.

[0066] In a fifth step 42, wrapped assembly is then placed into an ovenat a temperature above or nearly equal to the crystalline meltingtemperature of ePTFE. The wrapping applies pressure to regions of ePTFEthat are underlaid by raised portions of the textured mandrel. The ovenprovides the necessary heat to cause a strong ePTFE-ePTFE bond to formin these regions. The sintering time can vary from a few minutes to afew tens of minutes. The overall time depends to some extent on the massof the mandrel. If the mandrel is solid, it may take a considerable timefor the surface of the mandrel to reach sintering temperatures. Theprocess can be speeded up by using a hollow mandrel or even a mandrelcontaining a heating element so that the ePTFE is rapidly brought to asintering temperature. A thermistor or similar temperature sensor isadvantageous embedded into the surface of the mandrel so that it ispossible to determine when the ePTFE reaches sintering temperature. Inthis way the process can be accurately timed.

[0067] In the final step 44, the tape is removed from the mandrel (aftercooling) and the finished device is removed. Results in this stepindicate the success of the sintering step 42. If sintering time ortemperature is excessive, there may be some bonding of the PTFE tape tothe stent device. The solution is to reduce the sintering time and/ortemperature in future sintering. This is one reason that time,temperature and wrapping force should be carefully controlled. Thisproblem can also be avoided by using means other than PTFE wrapping toapply pressure to the device during the sintering process. At firstglance it would appear that the radial pressure can be applied by a“clam shell” heating device that clamps around the stent device andmandrel. However, such a device is not capable of applying even radialpressure. One possible solution is to divide the clam shell into anumber of segments, preferably at least six, each of which is equippedwith pressure means to force the segment radially towards the center oftextured mandrel. Similarly, the mandrel can be divided into segment orotherwise be capable of an increase in diameter (e.g. by formation froma material having a large coefficient of expansion upon temperatureincrease) to create radial pressure between the surface of the mandreland the surrounding clamshell.

[0068] An additional method of achieving bond pressure without wrappingis to use a clamshell having an inner surface relief mirroring thetextured mandrel. That is, there would be ridges and valleys that wouldexactly register with the ridges and valleys on the mandrel when theshell is closed. Similarly, a flat surface could be provided with ridgesand valleys matching the mandrel surface if that surface were unrolledonto a flat plane. With such a surface it is possible to roll themandrel in contact and registration with the flat pattern so thatdefined pressure is applied to the raised mandrel regions. The downwardforce applied to the mandrel controls the bond pressure while the rateof rolling controls the time a given bond is under pressure. Thisprocess can be carried out in an oven or the mandrel and surface cancontain heating elements. One method of ensuring registration betweenthe mandrel pattern and the flat surface pattern is to have gearsattached to one or both ends of the mandrel mesh with a toothed rackthat runs along one or both edges of the patterned surface. Contactpressure is controlled by weight of the mandrel or by a mechanicallinkage that applies a controlled downward force to the mandrel.

[0069] To this point no mandrel patterns or textures have beendescribed. It will be clear to one of skill in the art that thisinvention permits a complex pattern wherein the entire stent structureis mirrored by the valleys and ridges of the mandrel with the structuralmembers of the stent fitting into the valleys and the apices of theridges or raised portions falling at discrete points within the openareas of the stent. What may be somewhat less obvious is that farsimpler patterns can also produce excellent results in the presentinvention. One simple mandrel design is a “splined” mandrel wherein themandrel has a number of longitudinal ridges (splines) so that across-section of the mandrel looks something like a toothed gear. FIG. 2shows a perspective view of such a mandrel 20 with longitudinal splines22. FIG. 3 shows a cross section of the mandrel 20 wherein it isapparent that the splines 22 have rounded edges to avoid damaging orcutting the surface of the ePTFE.

[0070]FIG. 4 shows a perspective view of an encapsulated stent 30 madeon the splined mandrel 20. The stent 46 is composed of struts 48arranged in a diamond pattern. Regions 52 at the ends of the device(marked by cross-hatching) have complete bonding between the two-ePTFEtubular members. This region is produced by smooth, non-splined regionsof the mandrel. Dotted lines 54 marks the position of the splines andthe resulting regions of selective bonding. That is, the device hasspaced apart bonded regions running the length of the open diamondregions 56. Because of this orientation successive tiers of diamondregions 56 along the longitudinal axis of the device are alternatelybonded and unbonded. FIG. 6 shows a scanning electron micrograph of anoblique section through a longitudinally selectively bonded stent 44. Across-section of the strut 48 is shown as well as a bonded region 54 andan unbonded slip pocket 62. The unbonded pockets 62 allow free movementof the stent struts 48. However, even those diamond regions 56containing bonds 54 allow relatively unimpeded movement of the struts 48because the bond 54 is only down the central part of the diamond region56—relatively distant from the struts 48. Tests show that theselectively bonded stent 30 can be radially compressed with considerablyless force than a stent that is encapsulated by uniformly bonding allregions were the ePTFE tubular members contact each other. Thelongitudinal bonds somewhat restrict longitudinal compression of thedevice as the bonded regions buckle less readily than unbonded ePTFE.

[0071] The longitudinal bonds 54 do restrict the side to sideflexibility or bendability of the device to some extent. In someapplications this stiffening of the device is desirable while in otherapplications one needs a stent device that is able to bend more freely.Increased lateral flexibility can be achieved by using a mandrel withradial ridges rather than longitudinal ridges as shown in FIG. 7. Againthe ridges 58 are spaced apart in relation to the strut 48 spacing inthe stent to be encapsulated. If the stent 46 shown in FIG. 4 is used,the radial ridges 58 can be spaced apart to place circumferential bondsthrough alternate tiers of diamond regions 56. The resulting device ismore bendable laterally than the version with longitudinal bonds. Inaddition, the circumferential bonds result in a device that is moreeasily compressed longitudinally.

[0072] It is clear that the area and orientation of the bond regionsinfluence the properties of the final device. For example, a helicalpattern of ridges produces a device with intermediate properties: it ismore laterally bendable that the longitudinally bonded device of FIG. 4,but it has more resistance to longitudinal compression than does adevice with circumferential bonds. The pitch of the helical patterncontrols the overall effect with shallow pitches acting more likecircumferential ridges and steep pitches acting more like longitudinalridges. Multiple helices can be used with opposing (e.g., clockwise andcounter clockwise) producing a device that is more resistant to lateralbending. Virtually any combination of the described patterns can be usedto produce devices having a preferred direction of bendability ordevices that resist longitudinal compression in one region whilepermitting such compression in another.

[0073] The stent device illustrated in the above-figures is one in thestent struts form courses or diamond-shaped spaces in which the strutscontinue from course to course to create an extended tubular device.Stents are also available which consist of only a single course (orsegment) of diamond-shapes. The current method can advantageously beused to combine a number of these segments together to make an extendedtubular device. Frequently these single segment stents consist of analternation of larger and smaller diamond shapes. For example, thesegments can be arranged with large diamonds touching large diamonds.Other arrangements included a “twisted” design wherein each successivesegment is rotationally offset and an “alternating” design whereinalternate segment are rotated so that a given large diamond is boundedon either side by a small diamond. The precise properties of theresulting encapsulated device depend on these factors. However, thesignificant thing about the prior art encapsulation is that it produceda device that is relatively stiff and unbending.

[0074] Various adhesives (as opposed to directly adhering PTFE to PTFE)can also be used to create the pattern of bonded regions. FIG. 8 shows adiagram of one method for using adhesives to create selective bonds. Ina first step 32 a tubular graft member is placed on a support such as amandrel. In a second step 34 a stent (or stents) is placed over thefirst graft member. In the third step 64 a coating of adhesive is placedover the stent graft combination. This adhesive is one that is“activatable” meaning that the material is not inherently sticky as itis applied. However, it can be activated by applying heat, light or someother energy so that it hardens or otherwise changes to form a permanentbond. In the next step 64 a second tubular member is placed over theadhesive-coated stent. In the final step 66 a pattern of desired bondsis inscribed on the device with, for example a laser or a heated probeor a photolithographic mask image. The inscribing process providesenergy to local regions of the structure to activate the adhesive andcreate selectively bonded regions. A number of different activatableadhesive materials can be used in the present invention. One suchmaterial might be a layer or coating of a thermoplastic such aspolyethylene. This material can be activated by heat that melts it sothat it flows into the pores of the ePTFE. After cooling the plastichardens so that the PTFE of one tubular member is bonded to the othertubular member.

[0075]FIG. 9 shows a second adhesive-based method of creating selectivebonds. The initial steps are the same as in the previous method.However, in step 68 the adhesive material is applied selectively to formthe future pattern. This can be done, for example, by a screening oroffset printing method. An inherently sticky adhesive can be used or anactivatable adhesive (as in the previous method) can be employed. Thesecond tubular member is applied (step 36) and the adhesive pattern isformed either by applying pressure (when using an inherently stickyadhesive) or by applying pressure followed by an activation step—forexample heating to melt a thermoplastic adhesive.

[0076] The words used in this specification to describe the inventionand its various embodiments are to be understood not only in the senseof their commonly defined meanings, but to include by special definitionin this specification structure, material or acts beyond the scope ofthe commonly defined meanings. Thus if an element can be understood inthe context of this specification as including more than one meaning,then its use in a claim must be understood as being generic to allpossible meanings supported by the specification and by the word itself.The definitions of the words or elements of the following claims are,therefore, defined in this specification to include not only thecombination of elements which are literally set forth, but allequivalent structure, material or acts for performing substantially thesame function in substantially the same way to obtain substantially thesame result.

We claim:
 1. A method of making an endoluminal stent-graft, comprisingthe steps of: placing a first covering member composed of abiocompatible polymer on a surface having a pattern of elevated regions;placing a radially expandable stent over said first covering member inalignment with said pattern with said pattern; placing a second coveringmember composed of a biocompatible polymer over said endoluminal stentdevice; applying pressure to force said first covering member and saidsecond covering member into intimate contact through openings in thestent and in registration with the pattern; and heating the coveringmembers to form a pattern of bonds between the covering members, saidpattern of bonds corresponding to the pattern of elevated regions. 2.The method of claim 1 , wherein the radially expandable stent isselected from the group of stents consisting of balloon expandable, selfexpanding and memory shape stents.
 3. An encapsulated stent-graftproduced by the method of claim 1 .
 4. The method of claim 1 , whereinthe biocompatible polymer is polytetrafluoroethylene.
 5. The method ofclaim 1 , wherein the biocompatible polymer is further contains abioactive substance.
 6. A method of making an endoluminal stent-graft,comprising the steps of: placing a first covering member composed of abiocompatible polymer on a surface; placing a radially expandable stentover said first covering member; laying down an adhesive material overthe first covering member and the stent, the adhesive material forming apattern with regions of the first covering member and the stent beingfree of adhesive; placing a second covering member composed of abiocompatible polymer over the stent; applying pressure to force saidfirst covering member and said second covering member into intimatecontact through openings in the stent; and treating the stent-graft toform a pattern of adhesive-based bonds between the covering memberswhich pattern allows portions of the stent to move freely when the stentexpands.
 7. An encapsulated stent-graft produced by the method of claim6 .
 8. The method of claim 6 , wherein the radially expandable stent isselected from the group of stents consisting of balloon expandable, selfexpanding and memory shape stents.
 9. The method of claim 6 , whereinthe biocompatible polymer is polytetrafluoroethylene.
 10. The method ofclaim 6 , wherein the biocompatible polymer is further contains abioactive substance.
 11. A method of making an endoluminal stent-graft,comprising the steps of: placing a first covering member composed of abiocompatible polymer on a surface; placing a radially expandable stentover said first covering member; laying down a layer of an activatableadhesive material over the first covering member and the stent; placinga second covering member composed of a biocompatible polymer over thestent; inscribing a pattern of electromagnetic energy on the stent-graftto activate the adhesive forming a pattern of bonds between the coveringmembers which bonds corresponds to the pattern of electromagnetic energyand which bonds allows, portions of the stent to move freely when thestent expands.
 12. An encapsulated stent-graft produced by the method ofclaim 11 .
 13. The method of claim 11 , wherein the radially expandablestent is selected from the group of stents consisting of balloonexpandable, self expanding and memory shape stents.
 14. The method ofclaim 11 , wherein the biocompatible polymer is polytetrafluoroethylene.15. The method of claim 11 , wherein the biocompatible polymer isfurther contains a bioactive substance.